#include <iostream>
#include <vector>

using namespace std;

string test(string &str1, string &str2)
{
    cout << "str1 = " << str1 << endl;
    cout << "str2 = " << str2 << endl;
    string res;
    res = str1 + str2;
    cout << "str1 = " << str1 << endl;
    cout << "str2 = " << str2 << endl;

    return res;
}

int main()
{
    string str;
    string str1("abc");
    string str2("def");
    str = test(str1, str2);

    // int & a = 10;  非法 ????
    const int & a = 10;  // 坈法

    cout << "str = " << str << endl;
}